(1) Field of the Invention
The present invention relates to a heat-resistant liner-provided vessel closure and a process for the preparation thereof. More specifically, the present invention relates to a heat-resistant liner-provided vessel closure which can resist hot filling of a content and heat sterilization after filling and sealing and in which leakage caused by thermal deformation or thermal shrinkage is prevented, and also to a process for the preparation of this vessel closure.
(2) Description of the Related Art
A vessel closure provided with a liner of an olefin resin such as low-density polyethylene formed on the inner face side of a top plate of a vessel closure shell has been widely used as a vessel closure having excellent sanitary characteristics and sealing characteristics. However, in the case where a heat treatment such as heating sterilization is carried out after filling of a content, excessive reduction of the opening torque and stress cracking are caused.
As the means for solving this problem, Japanese Unexamined (Kokai) Patent Publication No. 58-183458 proposes the use of a hydrogenation product of a block copolymer of an alkenyl aromatic hydrocarbon (such as styrene) and a conjugated diene (such as butadiene) or a blend of this hydrogenation product and a polyolefin for the liner of a vessel closure.
This blend of the hydrogenated block copolymer and polyolefin is advantageous in that reduction of the opening torque after the heat treatment is small and stress cracking resistance is improved, but this liner is still insufficient in the heat resistance, especially the resistance to leakage caused by thermal deformation or thermal shrinkage of the vessel opening.
More specially, the blend according to the conventional technique can resist a heat treatment conducted at 80.degree. C. for 20 minutes, but it has been found that leakage is caused at a considerable frequency when a content is hot-filled at 95.degree. C. and a shower treatment is carried out for 3 minutes with hot water maintained at 90.degree. C.
The reason has not sufficiently been elucidated, but it is construed that although the hydrogenated block copolymer is relatively excellent in the heat resistance among thermoplastic elastomers, the compression permanent strain or permanent elongation in the above-mentioned high-temperature region is large and a divergence that cannot be neglected is produced between the vessel mouth and the vessel closure by thermal deformation or thermal shrinkage, with the result that fine spaces are formed in the sealed portion.